Rails: Switches Serializer according to STI Type

Introduction

Hello, here is Tamura.

Suppose you have model data of multiple types using STI and want to mix and return them in a single API endpoint. In this case, you would like to switch the serializer according to the STI type.

In this article, I will show you how to do this.

Issue

Suppose the following class hierarchy is realized in STI

SimplePosts has a title and body field, and QAPosts has a question and answer field. They all have a common posted_at field.

Suppose you define an API endpoint called /api/v1/posts and want to return JSON as follows:

Depending on whether the Posts type is SimplePost or QAPost, the JSON keys will differ.

If you are using ActiveModel::Serializers to output JSON, you will need to switch the Serializer depending on the type of Posts.

Solution

The code is shown below.

app/serializers/post_serializer.rb

app/serializers/simple_post_serializer.rb

app/serializers/qa_post_serializer.rb

app/controllers/api/v1/posts_controller.rb

Override the attributes method in PostSerializer. Since each data is taken as an object, the class is determined in case, and SimplePostSerializer.new(object).attributes(nil, reload) and QAPostSerializer.new(object).attributes( nil, reload) generates attributes and merges them into @attributes.

Reference: https://github.com/rails-api/active_model_serializers/blob/0fbe0fad0dec9368e9335b6280a46ca13442727e/lib/active_model/serializer.rb#L334-L343

The reason for overriding the json_key method is to display posts in the json toplevel object. If this is not defined, qa_posts and the type first processed by the Serializer will be set as the key, as shown below.

Note that if jsonapi_include_toplevel_object is false, there is no need to override the json_key method.

Reference: https://github.com/rails-api/active_model_serializers/blob/0fbe0fad0dec9368e9335b6280a46ca13442727e/lib/active_model/serializer.rb#L384-L392

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